Cesar Azpilicueta's inch-perfect cross found Alvaro Morata with all the time in the world to head past David de Gea into the top-right corner.
It was a fantastic header that capped what has been a phenomenal start to life at Chelsea in the Premier League for Morata.
The Spaniard now has seven league goals and is just one behind Sergio Aguero and Harry Kane (both eight) in the top goalscorers chart.
However, while a fine header, questions needed to be asked of United's defending.
Morata was completely free when Azpilicueta crossed for him, meaning he wasn't challenged and was able to pick his spot.
Centre-backs Phil Jones, Eric Bailly and Chris Smalling were nowhere to be seen, as the video below of Morata's goal shows.
MORATA'S GOAL VS MAN UTD
Jose Mourinho will inevitably be furious with his defenders for not marking Morata, who has shown this season just how deadly he is in the air.
Rio Ferdinand was certainly angry with the defending he saw from his former employers and pretty much nailed his reaction to the goal on Twitter.
In the tweet below, Ferdinand says that Jones and co. were too far away from Morata to even lay a hand on him, let alone challenge for the ball.
FERDINAND LAYS INTO MAN UTD'S DEFENCE
He wrote: "What a header....@ManUtd defenders not about to lay a hand on moratta let alone challenge him. Where's @AnthonyMartial at?!"
Wouldn't have happened in Ferdinand's day. The former centre-back also asked why Anthony Martial wasn't on the pitch and it would appear Mourinho heard his cries.
The Portuguese replaced Henrikh Mkhitaryan with Martial eight minutes after Morata's goal, but it wasn't enough for United as they slumped to a 1-0 defeat.